Crystal structures porous catalysts

Many but not all catalysts are porous materials in which most of the surface area is internal. It is sometimes convenient to speak of the structure and texture of such materials. The structure is defined by the distribution in space of the atoms or ions in the material part of the catalyst and, in particular, by the distribution at the surface. The texture is defined by the detailed geometry of the void space in the particles of catalyst. Porosity is a concept related to texture and refers to the pore space in a material. With zeolites, however, much of the porosity is determined by the crystal structure. 

Zeolites are crystalline aluminosilicates that are used as catalysts and that have a particular chemical and physical structure (Decroocq, 1984 Goursot et al., 1997). They are highly porous crystals veined with sub-microscopic channels. The channels contain water (hence the bubbling at high temperatures), which can be eliminated by heating (combined with other treatments) without altering the crystal structure (Occelli and Robson, 1989). 

Chromium has a large contribution to the MOF arena due to the fact that it has created the largest pore-sized metal catalysts yet known. Ferey created a crystal structure for porous chromium terephthalate, MIL-101, with very large pore sizes and surface area. Its zeotype cubic structure has an enormous cell volume ( 702 000 A ), a hierarchy of extra-large pore sizes (30 to 34 A), and a Langmuir surface area for N2 of 5900 300 m This... 

Fig. 15.25 Pathways for future electrocatalyst development for automotive PEMFCs. (a) Thick films or bulk single crystal and polycrystalline catalysts that are ideal for fundamental studies on surface structure and mechanisms these materials need to be modified into (c) and (d) to be applicable to fuel cells, (b) Typical commercial nanoparticles (2-4 nm) on a high-surface-area carbon support used in fuel cells at this time (c) Thin continuous films of catalyst on a support such as carbon nanotubes that may provide a physical porous structure for mass transport in a fuel cell (d) Core-shell catalysts where only the shell eonsists of precious metals and are supported on a typical high-surface-area support [72, 77, 89]...

A porous catalyst pellet usually consists of a close packing of small porous catalyst particles of size 10—1000 A. The gases diffuse in the empty space between the particles as well as in the microscopic pores of the particles. The most important physical property of such a structure is the area per unit mass of the pellet, denoted by Sp. The values of Sp are in the range of 1 —1000m /g. The mass per unit of total volume Pp, and the void fraction Sp are also essential for the physical characterization of the catalyst. In many cases the porous pellets are impregnated with a solution of a metal salt and subsequently heated or treated chemically. The microscopic crystals of the metal or metal oxide deposited on the porous surface by this process give the catalyst its chemical reactivity. 

The formation of a solid catalyst is a complex process depending on the chemical properties of the solid and on its porous and crystal structures. These different aspects have also been studied, often by X-ray diffraction, giving thus access to the size and organisation of the microcrystals. The activity of the solids is related to the decrease in size of the elemental crystals 22-24) in a previous study, we have established the particular pore structure corresponding to active catalysts and how it depends on the grinding process. ... 

Surface scientists study the surface of the catalyst in order to obtain information that can be correlated with the kinetics and mechanisms of the catalytic reactions. In this way, they hope to be able to improve the activity, selectivity and lifetimes of the catalysts. Real catalysts with porous supports with large surfaces have very complex structures that can hide the metal sites, however. The more simple and rational approach uses crystals of the catalyst. In section 2.4, the dramatic influence of... 

One of the major criticisms of MOFs with respect to their use as heterogeneous catalysts is their lower crystal stability compared with that of zeolites and other porous aluminosilicates [21]. In fact, it is well known that some of the first MOFs reported such as MOF-5 are notoriously instable and the crystal structure tends to collapse on storage under ambient conditions or during the reaction [22]. This lack of stability seems to be relatively common for Zn- and Cu-containing MOFs. For instance, Cu3(BTC)2 (BTC 1,3,5-benzenetricarboxylate) is unstable in the presence of thiols, probably due to the strong... 

In summary, large (>lpm) single crystal platelets of aurichalcite produced highly dispersed Cu and ZnO particles with dimensions on the order of 5 nm, as a result of standard catalyst preparation procedures used in the treatment of the precipitate precursors. The overall platelet dimensions were maintained throughout the preparation treatments, but the platelets became porous and polycrystalline to accommodate the changing chemical structure and density of the Cu and Zn components. The morphology of ZnO and Cu in the reduced catalysts appear to be completely determined by the crystallography of aurichalcite. 

Since the initial work of Onto et al. (1) a considerable amount of work has been performed to improve our understanding of the enantioselective hydrogenation of activated ketones over cinchona-modified Pt/Al203 (2, 3). Moderate to low dispersed Pt on alumina catalysts have been described as the catalysts of choice and pre-reducing them in hydrogen at 300-400°C typically improves their performance (3, 4). Recent studies have questioned the need for moderate to low dispersed Pt, since colloidal catalysts with Pt crystal sizes of <2 nm have also been found to be effective (3). A key role is ascribed to the effects of the catalyst support structure and the presence of reducible residues on the catalytic surface. Support structures that avoid mass transfer limitations and the removal of reducible residues obviously improve the catalyst performance. This work shows that creating a catalyst on an open porous support without a large concentration of reducible residues on the Pt surface not only leads to enhanced activity and ee, but also reduces the need for the pretreatment step. One factor... 

These data have been verified by electron microscope photographs Fig. 11 shows an example for a catalyst reduced at 500° C. Clearly, the carrier forms a large porous structure, in which the small nickel crystals find a place. 

Figure 8. SEM surface images of partly crystallized sections of an activated Fe Zr alloy used for ammonia synthesis [23, 24J The main image reveals the formation of a stepped iron metal structure with a porous zirconium oxide spacer structure An almost ideal transport system for gases into the interior of the catalyst is created with a large metal-oxide interface which provides high thermal and chemical stability of this structure The edge contrast in the 200 keV backscatlered raw data image arises from the large difference in emissivity between metal and oxide It is evident that only fusion and segregation-crystallization can create such an interface structure.

The catalyst particle is usually a complex entity composed of a porous solid, serving as the support for one or more catalytically active phase(s). These may comprise clusters, thin surface mono- or multilayers, or small crystallites. The shape, size and orientation of clusters or crystallites, the extension and arrangement of different crystal faces together with macrodcfects such as steps, kinks, etc., are parameters describing the surface topography. The type of atoms and their mutual positions at the surface of the active phase or of the support, and the type, concentration and mutual positions of point defects (foreign atoms in lattice positions, interstitials, vacancies, dislocations, etc.) define the surface structure. 

The aerogel-prepared metal oxide nanoparticles constitute a new class of porous inorganic materials because of their unique morphological features such as crystal shape, pore structure, high pore volume, and surface areas. Also, it is possible to load catalytic metals such as Fe or Cu at very high dispersions on these oxide supports and hence the nanocrystalline oxide materials can also function as unusual catalyst supports. Furthermore, these oxides can be tailored for desired Lewis base/Lewis acid strengths by incorporation of thin layers of other oxide materials or by preparation of mixed metal oxides. 

Taking into account the bimodal structure of the catalyst of this study, in which microporous crystals of zeolite are agglomerated with a binder (bentonite) and with alumina (inert charge), both of high mesopore proportion, the limitation to internal diffusion of oxygen in both regions (in series) of the porous structure has been quantified. 

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